The present invention relates to equipment for desalination of seawater by reverse osmosis.
Seawater reverse osmosis desalination systems have been used for obtaining fresh water from seawater in which the concentration of salt is typically 40,000 ppm. The seawater, also referred to as “fresh brine”, is passed over the reverse osmosis semi permeable membrane by a pump at high pressure, typically in the range of 20-120 bar. The percentage of fresh brine converted to fresh water (the “recovery ratio”) is typically on the order of 40%-70%. In order to reduce energy wastage, energy should be recovered from the remaining 30%-60% high pressure brine before the liquid is discarded.
One example of such an energy recovery system based on a double-piston end-to-end cylinder arrangement is disclosed in U.S. Pat. No. 4,637,783 to Andeen. High pressure spent brine delivered to one cylinder applies pressure to fresh brine on the other side of the piston to supply the fresh brine into the reverse osmosis membrane array. A difference in surface area between the two sides of the piston (due to the piston rod) results in slight pressure amplification. At the same time, the motion of the piston rod causes expulsion of the spent brine from the paired cylinder and draws in fresh brine to the other cylinder, ready for the reverse stroke.